A typical optically pumped, fluid cooled slab laser includes a slab of solid state lasing medium. Pumping means such as a lamp and reflector are situated to impinge optical radiation on the lasing medium in order to pump the atoms in the lasing medium to a metastable state. A cooling fluid is flowed across at least a portion of the lasing medium to remove heat generated therein by the optical pumping. A laser beam is subsequently produced by passing a beam of coherent light, either externally generated or stimulated within the lasing medium, oscillatingly through the lasing medium, the coherent light beam being amplified with each pass through the medium.
Such slab lasers typically exhibit both width-wise and thickness-wise wave-front distortions of the laser beam, the width-wise distortion being particularly prominent in areas proximate lateral edges of the lasing medium. This wave-front distortion diminishes the useable area of the lasing medium, the operating efficiency of the slab laser and the beam quality of the laser beam. Such wave-front distortion is mainly attributable to three known phenomena: (1) width-wise variation of the refractive index of the lasing medium caused by thermal gradients within the lasing medium, i.e. thermal lensing; (2) variations in the refractive index of the lasing medium due to a stress-optic effect caused by a nonuniform stress field in the lasing medium; and (3) beam distortion induced by a deformation of major faces of the lasing medium, caused by thermal expansion and compatibility requirement for a lasing medium of finite rectangular cross-section.
Commonly assigned U.S. Pat. No. 3,633,126 entitled "Multiple Internal Reflection Face-Pumped Laser" (Martin et al.), incorporated in its entirety herein by reference, addresses the problem of thickness-wise variation of the refractive index of the lasing medium. In Martin et al., the beam of coherent light is introduced into the lasing medium in an off-axial direction, such that each ray in the beam is multiply, internally reflected in the thickness dimension through regions of varying refractive indexes during each pass through the lasing medium. The varying refractive indexes encountered in the lasing medium thickness dimension and their distortional effects on the beam are thereby minimized.
U.S. patent No. (allowed U.S. patent application Ser. No. 914,431) entitled "Method and Apparatus For Compensating For Wave Front Distortion in a Slab Laser" (Azad) of common assignment and inventorship, which is incorpoated in its entirety herein by reference, addresses the problem of width-wise wave-front distortion. It is observed that the lasing medium exhibits a negative focal power lens effect in lateral edge portions in the width dimension thereof. The invention disclosed therein is directed to apparatus and method for introducing a positive focal power lens effect, approximately equal in magnitude to the negative focal power lens effect, into a central portion of the lasing medium width. Then, the coherent light beam is passed at least twice through the lasing medium and generally along a longitudinally extending lasing axis of the medium such that the rays of the coherent beam wavefront traverse alternate regions of positive and negative focal powers of equal magnitude. In this manner, the wave-front distortion due to variations in refractive index in the lasing medium width dimension are minimized.
The positive focal power lens effect disclosed in the Azad patent is created by shaping the reflector surrounding the pumping means so as to direct greater pumping energy into the central portion of the lasing medium width. The reflector shape is calculated to only apportion an additional amount of pumping energy to the lasing medium central portion sufficient to create a positive focal power lens effect approximately equal in magnitude to the negative focal power lens effect experienced at the lateral edge portions of the lasing medium. A method for computing a reflector shape suitable for accomplishing this end is disclosed in the above-incorporated U.S. patent No. [allowed U.S. patent application Ser. No. 9l4,43l] to Azad and in commonly assigned copending U.S. patent application Ser. No. [914,431, 10/03/86]. The present invention is directed to specific reflector shapes effective to introduce the appropriate positive lens effect into the lasing medium central portion.